WARNING
The source rod should always be in the SAFE
position when the gauge is not in use.
DENSITY
The Troxler Model 3430 Plus and 3440 Plus Surface Moisture-
Density Gauges use two modes of operation: direct transmission
mode (with the source rod extended into the material to be
measured) and backscatter mode (with the source rod in the
backscatter position, just above the surface of the material). Source
rod positions are described in Chapter 3.
In the direct transmission position, the source rod extends through
the base of the gauge into a pre-drilled hole to a desired depth.
Photons from the cesium-137 (Cs-137) source in the source rod pass
through the test material. While passing through the test material,
the photons collide with electrons and lose energy. A high material
density increases the probability of these photon collisions. This
decreases the number of photons that reach the Geiger-Mueller
(G-M) detectors in the base of the gauge. Thus, the number of
photons reaching the detectors is inversely related to the density of
the material: the higher the density of the material, the fewer the
photons that reach the detectors. Using the gauge calibration, the
gauge software converts the G-M detector counts to a density value.
In the backscatter position, the source rod is lowered to the first
notch below the SAFE (shielded) position. This places the source
and the detectors in the same plane, with the source just above the
surface of the material. Shielding between the source and detectors
greatly reduces the number of photons reaching the detectors in a
direct path from the source. Thus, the photons from the Cs-137
source must travel into the test material and scatter (or reflect) at
least once to reach the G-M detectors. The detectors in the gauge
base count these scattered photons.
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